RMs can be used to make optical films, like compensation, retardation or polarisation films, e.g. for use as components of optical or electrooptical devices like LC displays, through the process of in-situ polymerisation. The optical properties of the films can be controlled by many different factors, such as mixture formulation or substrate properties. The optical properties of the film can also be controlled by changing the birefringence of the mixture. This determines the necessary thickness for a given retardation at a particular angle as well as controlling the birefringence dispersion. High birefringence materials also give high birefringence dispersion whilst low birefringence materials give low birefringence dispersion.
There is a need to prepare anisotropic polymer films having a higher optical dispersion than that achieved with RM materials and mixtures as disclosed in the literature to date. In particular there is a need for anisotropic polymer films with the optical properties of a “negative C” film that can also have exceptionally high birefringence dispersion.
The dispersive power of an RM film can be defined in many ways, however one common way is to measure the optical retardation at 450 nm (R450) and divide this by the optical retardation measured at 550 nm (R550) to obtain the ratio R450/R550. In the case of a polymer film having the optical properties of a negative C plate, the on-axis retardation is substantially zero, and the retardation of the film becomes increasingly negative as the angle of measurement deviates from normal. The dispersive power of a negative C film does slightly change with measurement angle, and generally decreases with increasing angle. FIG. 1 shows a typical plot of retardation against wavelength, measured at an angle of +40° off normal, in this case for a polymerised negative C film made from the commercially available reactive mesogen mixture RMM482 (Merck KgaA, Darmstadt, Germany). The optical dispersion R450/R550 for this film is 1.081.
The origin of the retardation dispersion is due to the fact that the two refractive indices ne, no of the anisotropic molecules (wherein ne is the “extraordinary refractive index” in the direction parallel to the long molecular axis, and no is the “ordinary refractive index” in the directions perpendicular to the long molecular axis) in the anisotropic film change with wavelength at different rates, with ne changing more rapidly than no towards the blue end of the spectrum. One way of preparing a material with high retardation dispersion is to design molecules with increased ne dispersion whilst keeping no dispersion largely unchanged. The optics of coatable negative C RM films has been disclosed for example in WO 2004/013666 A1.
One aim of this invention is to provide polymerisable materials, especially RMs and mixtures thereof, that are designed to have a higher ne than is commonly found in commercially available RM materials, and can be processed into optically anisotropic polymer films, especially negative C films, having higher retardation dispersion compared to films made from presently available RM mixtures. Another aim of the invention is to extend the pool of RM materials available to the expert. Other aims are immediately evident to the expert from the following description.
It has been found that these aims can be achieved by providing reactive mesogenic compounds and mixtures as claimed in the present invention.
In particular, it has been found that by using RMs with a tolane group and a terminal —NCS group it is possible to provide polymerisable LC mixtures and polymer films with high optical dispersion.
U.S. Pat. No. 5,332,522 discloses chiral nematic copolymers comprising chiral and achiral mesogenic groups in the side chain, wherein the achiral mesogenic side chain may also comprise a tolane group and a terminal NCS group. However, this document does neither disclose nor suggest RMs according to the present invention, or their use in polymerisable LC materials or optical films with high optical dispersion.